1. Field of the Invention
The present invention relates to a vehicle suspension system, and specifically to a rear suspension system for a motor vehicle.
2. Description of the Prior Art
Conventionally, a vehicle rear suspension system, such as is shown in U.S. Pat. Nos. 4,754,992, 4,690,426, comprises a wheel carrying member for rotatably carrying a vehicle wheel, a plurality of lateral links extending outwardly in a transverse direction of a vehicle body and connected to the wheel carrying member at the outer ends thereof, and a trailing arm or longitudinal member extending in a longitudinal direction of the vehicle body and connected to the wheel carrying member at a rear end thereof so as to effect toe angle and camber angle controls of the wheel when the wheel is bumped to move upward relative to the vehicle body and rebounded to move downward relative to the vehicle body during running of the vehicle.
With this suspension system structure, the longitudinal member is normally connected with the wheel carrying member through a rigid connection to resist against a moment acting on the wheel under a braking operation. It has been also known in such a suspension system that the trailing arm is constituted by a plate member flexible in the transverse direction of the vehicle body. Also a busing member deformable in the transverse direction is provided in a joint of the longitudinal member pivotally connected to a body member of the vehicle to allow a toe angle change of the wheel under the bumping action and the rebounding action of the wheel. It should be noted, however, that these suspension system structures are disadvantageous because a desirable toe angle change can be hardly obtained in the wheel.
Under these circumstances, there has been proposed a suspension system in which the longitudinal member is connected at a rear end thereof with the wheel carrying member through a joint which allows a pivotal movement between the longitudinal member and the wheel carrying member about a vertical axis and restricts a displacement of the longitudinal member relative to the wheel carrying member along the vertical axis so that the longitudinal member bears the wheel against the moment acting thereon under a braking condition and the like and the wheel smoothly produces a desirable toe angle change in the bumping action and the rebounding action as shown in Japanese Utility Model Public Disclosure No. 61-59107, laid open to the public in 1986.
With the structure proposed in the Japanese Utility Model application, i.e., wherein the longitudinal member for bearing a force acting on the wheel in the longitudinal direction of the vehicle body is pivotally connected with the wheel carrying member, the joint between the longitudinal member and the wheel carrying member allows a rotative movement of the longitudinal member relative to the wheel due to a rearward force acting on the wheel under a braking operation. This movement may affect a toe direction of the wheel defined by the plurality of the lateral links of the suspension system. In this regard, when the brake is operated, the rearward force resulted from the braking operation acts on a wheel center. The joint between the longitudinal member and the wheel carrying member is located spaced from the wheel center forwardly and inwardly by a certain distance respectively in a horizontal plane including the wheel center. As a result, when a rearward force acts on the wheel center due to the braking operation, the rearward force produces a moment causing a change in a toe direction of the wheel.
Generally, the toe direction of a wheel, specifically that of a rear wheel, affects the running stability of the vehicle. Therefore, an arrangement of the lateral links is contemplated to optimize the toe direction of the wheel. It will be thus understood that the moment acting on the joint is undesirable in establishing an optimal toe direction of the wheel. In particular, a moment which causes the wheel to produce a toe angle change in a toe-out direction from a point of running stability of the vehicle under a braking condition should be avoided.